1. Field of the Invention
The present invention relates to a method of forming thin films on a substrate and, more particularly, to a sputtering method and apparatus having high controllability with respect to both the thickness and the quality of such film. 2. Description of the Prior Art
In microelectronics products such as integrated circuits (ICs) and thin film devices, a plurality of thin films are shaped on a substrate to form superposed layers for enhancing the performance of circuits and achieving a higher density thereof in such a structure, the respective characteristics of the layered thin films are mutually different depending on the functions required for the individual films, so that there exist great differences among both the thicknesses and the qualities of the individual layered films. For instance, the film thickness difference reaches 10 to 100 times in some cases.
As an example using such multilayer thin films, there is known a technique of metallization for soldering use. In order to prevent dissolution of a metallizing coat against solder, it is necessary to provide a film thickness of several microns in metallization for soldering use. According to a cited reference "Electron Components Conf." (Vol. 32, pp. 346-353, 1982), an exemplary metallizing coat for soldering use has a composition of titanium Ti (approx. 3000.ANG.) - palladium Pd (approx. 3000.ANG.) - gold Au (5 .mu.m). In this example, the films have wide differences among the respective thicknesses with the ratio thereof reaching about 20.
In application of such a metallizing coat to a thermal head or a TFT (Thin Film Transistor) active matrix substrate, sputtering is adapted to form a desired film due to the necessity of a large area. With regard to an in-linen sputtering apparatus, a conventional type is such that an intermittent sputtering controller 5 is removed from the apparatus illustrated in FIG. 1. In this apparatus, a Ti layer is formed by a first target, a Pd layer by a second target, and an Au layer by a third target, respectively. Since the movement speed is maintained constant due to the in-line system, the thickness ratio of the films formed on the substrate is determined by a combination of the target length L and the film forming speeds of the target materials.
The target length L is standardized, and varying the target size freely to change the film thickness ratio is not practically advantageous in the expensive sputtering apparatus, and therefore it is customary in actual manufacture to increase the film thickness ratio by arraying a plurality of targets of the length L. However, the allowable number of the targets to be arrayed is at most two or three since such an array brings about greater dimensions and higher cost of the sputtering apparatus.
Another technical factor relative to changing the film thickness ratio is a film forming speed. This speed is determined by the density of an electric power applied to the target and the sputter rate of the target material.
The sputter rates of the individual materials are Ti =0.5, Pd =2.0 and Au =2.5. For obtaining the aforementioned film thickness ratio 20 of Au to Pd, the power ratio applied to the targets needs to be set to 20. In this case, the required film thickness ratio cannot be achieved unless the sputtering power density applied to the target for the thinner film is set to an extremely small value of 1/5 to 1/50. Meanwhile, it is known that if the sputtering power density is so minimized as mentioned above, the film quality deteriorates and fails to attain the desired characteristics. For example, as disclosed in another cited reference "Journal of Applied Physics" (Vol. 44, No. 3, p. 1009, 1972), the particle size in the composition of the sputtered film is greatly dependent on the film forming speed. Such particle size needs to be well controlled since the etching characteristics and various properties relative to anticorrosion, antimigration, mutual diffusion and so forth are related to particle size. However, there has been a disadvantage heretofore that, if priority is given to the control of the film thickness as described, it becomes impossible to attain satisfactory control of the film quality.
Thus, in the prior art mentioned above, sufficient consideration is not given for ensuring adequate compatibility of the film thickness and the film quality. As a result the required quality or thickness of each film is unachievable.